- #1
vorcil
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Homework Statement
The Earth can be regarded as a sphere of radius R,
the volume density of charge distributed as [tex] \rho = \frac{\rho_s r}{R} [/tex]
where the density is 0 at thcce centre and rises linearly with radius until it reaches ps at the Earth's surface
i) prove that the total charge on the Earth is [tex] \pi \rho_s R^3 [/tex]
ii) use gauss's law to find an expression for E at the point r outside the Earth (r>R)
iii) use gauss's law to find an expression for E within the Earth's interior (r<R)
iv) write expressions for the x-, y- and z- components of E at a point outside the Earth and calculate [tex] \bf{\nabla .E }
Homework Equations
Qenclosed = [tex] \int_v \rho d\tau. [/tex] (for a volume)
[tex] \oint_S \bf{E} . d\bf{a} = \frac{1}{\epsilon} Qenclosed [/tex]
The Attempt at a Solution
[tex] \int_v \rho \dtau [/tex]
substituting in the given rho,
[tex] \int \rho_s \frac{r}{R} \dtau[/tex]
the outer radius r is = R so it just becomes R/R =1,
[tex] \rho_s *1 \int \dtau [/tex]
this is where I integrate the dtau and it should give me the volume of a sphere
[tex] \rho_s \frac{4}{3}\pi r^3 [/tex]
this sort of looks like the answer I'm looking for
HOWEVER I DONT KNOW HOW TO GET RID OF THE 4/3 constant!
what have I done wrong?
i've tried heaps of other methods, but I'm always getting some type of constant out the front